1. Field of the Invention
The invention relates to a smoothing roller in a printing unit of a rotary printing machine.
2. Description of the Related Art
Reference DE 44 24 913 A1 discloses a short inking unit for the inking of a form cylinder for dry offset printing. The short inking unit has an ink transfer roller and an ink applicator roller. The ink transfer roller is designed as a screen roller. Between the depressions on the outer surface of the screen roller there are webs which have high affinity to a separating agent, for example silicone oil. For this purpose, the hard-ceramic material of the ink transfer roller is impregnated with silicone oil, or silicone rubber or fluorosilicone rubber is embedded into the hard-ceramic material. The depressions are lined with an ink-friendly coating repelling the separating agent. The design of the ink transfer roller is intended to avoid ink transfer difficulties when using inks mixed with a separating agent.
Reference DE 44 24 920 A1 discloses a short inking unit which likewise serves for the inking of a waterless flat printing plate. In this case, a printing ink provided with a separating agent is applied to the form cylinder by a screen roller via two applicator rollers. One of the two applicator rollers is ink-carrying, while the other applicator roller is designed to be separating agent-friendly. High affinity to the separating agent is achieved by a coating having a silicone oil such, for example, as silicone rubber or fluorosilicone rubber.
Irrespective of whether a printing ink has to contain a separating agent in order to be particularly suitable for dry offset printing or whether printing is carried out using a dampening medium, there is the problem, particularly in the case of short inking units, of obtaining as smooth an ink film as possible on the form cylinder, in order to achieve high printing quality.
The object of the invention is to improve a printing unit of a rotary printing machine in such a way that an ink film of uniform thickness is obtained on a form cylinder.
This object is achieved by a smoothing cylinder applied to a form cylinder in a printing unit of a rotary printing machine or applied to an ink applicator roller, wherein an outer surface of the smoothing roller has an ink-repelling effect.
In a preferred embodiment, a transfer of printing ink onto the smoothing roller is prevented by appropriately designing the latter. The result of this, in particular, is that, in a nib between the form cylinder and the smoothing roller or between the ink applicator roller and smoothing roller, the ink film is merely squeezed and not split. Since the surface of the smoothing roller has an appropriately smooth design, the ink film thereby assumes a markedly more homogeneous structure, that is to say greater smoothness.
In a further embodiment, an improvement is obtained if the smoothing roller is not driven by friction with the ink applicator roller or form cylinder, but is provided with its own drive. The rolling conditions between the smoothing roller and ink applicator roller or form cylinder can thereby be improved for exerting an additional smoothing effect on the ink film. The ink-repelling effect of the smoothing roller may be achieved by coating the smoothing roller with a material which is coordinated with the printing ink used. In the case of waterless offset printing, this coating may include a silicone coating, such as is present in the print-free regions of the printing form.
The ink-repelling effect may also be achieved or assisted by applying a liquid to the smoothing roller during operation of the inking unit. In this case, the liquid thoroughly wets the smoothing roller and thus prevents ink from being applied to the latter. In wet offset printing, it is possible to use a liquid for the ink-repelling effect which has a composition similar to that of the dampening medium or is the dampening medium itself. In the case of dry offset printing, a silicone oil or an appropriate silicone oil substitute such, for example, as is mixed in the printing inks for waterless offset printing may be used.
Furthermore, the ink-repelling smoothing roller may, in the case of dry offset printing, comprise material or a coating having an affinity to silicon oils for absorbing the silicone oil fractions from the printing ink. The smoothing roller with this property forms a film which leads to a uniform ink film on the ink applicator roller or form cylinder. In addition, when the smoothing roller is applied onto an link applicator roller, the affinity of the smoothing roller to silicon oil causes the absorption of excess silicone oil and leads to a uniform distribution of the silicone oil fraction in the ink film. The silicone oil excess is absorbed by the smoothing roller and is redistributed to other regions of the ink applicator roller. The same likewise applies accordingly to the silicone oil substitute or when dampening medium has previously been mixed into the printing ink.
The smoothing roller is set axis-parallel to the ink applicator roller or the form cylinder, but may also be set obliquely, with the result that additional distribution of the printing ink layer occurs.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.